In a semiconductor integrated circuit (IC), such as a banking smartcard, the IC is vulnerable to a breach of security if it falls into the hands of a dishonest person. The IC may be reverse engineered in order to reveal or modify functions and confidential data contained therein. It is known that such IC's have been decapsulated and have even undergone depassivation of the upper protective layer.
U.S. Pat. No. 4,952,796 describes a circuit which comprises a current generator delivering current which flows into a reversed biased transistor junction. If subject to light, the reverse current in the junction increases, and the voltage at the junction terminals drops.
A problem with this arrangement is that incident light will generate reverse currents in transistors 11 and 2 of FIG.1, and this may affect the voltage drop detected at the output. Furthermore, the biasing and current generation functions of the above circuit are more susceptible to manufacturing process variations.
Also current drain, particularly in a smartcard, should be kept to a minimum, and the reverse current adversely affects the current consumption of the device. Lastly, the above circuit takes up much valuable semiconductor area, and again this is disadvantageous in a smartcard, where demand for space is at a premium.
This invention seeks to provide a light detection device which mitigates the above mentioned disadvantages.